FUNDAMENTALS OF EARTH SCIENCE I

IGNEOUS ROCKSFALL SEMESTER 2018 Where do igneous rocks form?

Understanding Earth 6th Ed.

 Classification of igneous rocks
1. TEXTURE

Cooling of magma/lava
Crystallization of minerals
Formation of an igneous rockSlow coolingwithin thelithosphere

Rapidcoolingnear or on Earth’s
surfaceINTRUSIVE IGNEOUS ROCKS
Coarser-grained texture

EXTRUSIVE IGNEOUS ROCKS
Finer-grained texture

e.g. granite

e.g. basalt

Understanding Earth 6th Ed.

Different types of igneous rocks identified based on the texture

Understanding Earth 6th Ed.

2. CHEMICAL AND MINERALOGICAL COMPOSITION
Felsic vs. mafic compositions
Felsic : feldspar-silica

Mafic : magnesium-ferric

FELSIC

MAFIC

Igneous rocks enriched in SiO2 andsilicates rich in Al, K, Na
Quartz (SiO2)
Orthoclase (K-rich feldspar)
Plagioclase (Na/Ca-rich feldspar)
Muscovite (K-rich mica)
Example: granite (continental crust)Light color

Igneous rocks enriched in silicatesrich in Fe, Mg
Biotite (mica)
Amphibole group
Pyroxene group
OlivineExample: basalt (oceanic crust)
Dark color

CONTINUUM

Pyroxene

Quartz:SiO2

Anorthite:
CaAl2Si2O8
Ca-rich plagioclase

Basalt0-7 kmFe, Mg >>GraniteOrthoclase:
KAlSi3O8

Oceanic crust Fe, Mg > 3.0 g/cm3

Continental crust35-60 km

3.4 g/cm3

Mantle2.8 g/cm3

Peridotite

Olivine: (Mg, Fe)2SiO4

Al, K, Na >Pyroxene: XY(Si,Al)2O6
Enstatite (MgSiO3) andferrosilite (FeSiO3)
Augite(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6

DOMINANT IN
EARTH’S UPPER
MANTLE

Intrusive
Extrusive

ABUNDANT IN
THE CONTINENTAL
CRUSTAnorthite
CaAl2Si2O8
KAlSi 3O8

ABUNDANT IN THEOCEANIC CRUSTSiO2NaAlSi3O8Albite(underlying ocean floor)(Mg, Fe)SiO 3(Mg, Fe)2SiO4

LIGHT color

DARK color

Crystallize and melt at
different T !!!

http://www.gso.uri.edu/lava/MagmaProperties/properties.html

Understanding Earth 6th Ed.

Production ofbasaltic magmas atoceanic hot spots

Production of more
felsic magmas atocean-continentsubduction zones

Low-viscosity
magmasHigh-viscosity
magmas (+ rich involatiles)

Mafic oceanic
crust

Felsic continental
crust

Melting ofmantle rocks

CaCO3 + SiO2 → CaSiO3 + CO2

LOW RISK OF EXPLOSION

H2O-rich
CO2-rich

HIGH RISK OF EXPLOSION

Basaltic volcanism (mafic composition)
Shield volcanoes“PAHOEHOE” LAVA

Hawaii hot spot“AA” LAVA

www.britannica.com

Understanding Earth 6th Ed.

“PILLOW” LAVA

www.britannica.com

Understanding Earth 6th Ed.

Rhyolitic volcanism (felsic composition)
Volcanic domes
MOUNT CHAITÉN (Chile)

RHYOLITIC LAVA DOME (Oregon, USA)Sam Beebe (Wikipedia)

Understanding Earth 6th Ed.

USGS Processes of magma formation
Factors controlling magma production:
1. Temperature• All minerals do not melt at the same temperature (felsic vs. mafic)

2. Pressure• Lower pressures result in lower melting temperatures→ Lowering the pressure facilitates melting!

3. Water content• Increased water content results in lower melting temperatures→ Adding water facilitates melting!

1. Role of temperature
Different minerals melt at different temperatures
Parent rock of identicalcomposition

MAGMA 1MAGMA 2

T1

T2

Temperature 1 (T1) < Temperature 2 (T2)
% melting 1 < % melting 2
Composition of partial melt 1 (magma 1) = composition of partial melt 2 (magma 2)

2. Role of pressure••High pressures deep in the Earth’s interior prevent rocks from melting
Hot mantle rock begins to melt when it rises beneath mid-ocean ridges andhotspots when the pressure drops =Decompression melting

Mid Ocean Ridge(MOR)

Decompression
melting

Ascending hot
mantle rock

HOT SPOT

Decompression
melting

Ascending hot
mantle rock

3. Role of water
Sedimentary rocks carried by the subducting plate have a high watercontent in the open space between grains (pores) and in clay minerals.
Water-induced melting"Cold" subducting
oceanic lithosphere

Subduction

Lithosphere
LithosphereHigh water content triggers
melting at relatively lowtemperature

Water molecules
disrupt chemical bonds
and lower the melting
temperature of silicate
minerals.

Hydrous Aluminium Phylosilicates (smectite group)e.g. montmorillonite → (Na, Ca)0.33(Al, Mg)2Si4O10(HO)2.nH2O

Mg2+,Na+,Ca2+, H2O

Understanding Earth 6 th ed.

How magma forms: Geothermal gradient & rock solidus
Red line (geotherm):
Green line (solidus):solRock temperature vs. depth (T increases with depth)
Temperature at which the rock starts to melt

liq

DECOMPRESSION MELTING

WATER-INDUCED MELTING

Oceanic crust
Oceanic lithosphere
MantleAsthenosphere (main originalsource of magma)
http://www.ei.lehigh.edu/learners/tectonics/heatflow/heatflow2.html

Magma is less dense than surrounding solid rocks and rises throughfissures in the rock or by melting its way up.
Magma accumulates in large magma chambers in the crust.

Understanding Earth 6th Ed.

Forms of igneous intrusions
1.

Magma chamber (pluton)

2.

Surrounding rocks melt
and influence magmacomposition3.

Different minerals
crystallize at different
temperatures which
influence the compositionof the remaining melt

4.

Sill (horizontal sheet-likeintrusion)5.

Dyke (vertical sheet-likeintrusion)

6.

Central vent

7.

Side dent

8.

Lava flow

9.

Pyroclasts Magma crystallization and formation of igneous rocks
How can we explain the diversity of igneous rocks?
1. Crystal fractionation•Minerals crystallize at different temperatures. Minerals that crystallize firstin the magma chamber tend to settle down first. This is called crystal
fractionation. This process results in the formation of igneous rocks ofdifferent compositions. One single parent magma can therefore produce
different igneous rocks. Change in magma composition during
crystallization is called magmatic differentiation.

2. Crustal contamination•

Changes in magma composition as the magma travels in the crust andincorporate pieces of the surrounding crustal rocks.

3. Magma mixing•The mixing of magmas with different chemical compositions may lead tothe formation of igneous rocks whose compositions differ from the rocks
that would have been produced if the two magmas had crystallized
separately without mixing.

1. Crystal fractionation
The Bowen’s reaction series (established experimentally)

Understanding Earth 6th Ed.

Minerals crystallizing first tend to settle down first in magmatic intrusions
which means that layers of igneous rocks of different compositions can form.
Crystal settling rate also depends on density and size of crystals and theviscosity of the remaining magma (+ turbulences in magma chamber)
Order that
follows Bowen’sreaction series

32
1

Olivine crystallized first and settled down at bottom
Understanding Earth 6th Ed.

2. Crustal contamination
Mafic to felsic

Mafic

Mafic tointermediate

Contamination by
sedimentary rocks andcontinental crustal rocks

Magma derived frommantle rocks

Contamination by
oceanic crustal rocks(including sediments)

Original magma derived mostly from mantle rocks
ULTRAMAFIC

3. Magma mixing

NB: Some magmas are immiscible, which means they cannot mix (like oil and water)
Understanding Earth 6th Ed.

Igneous differentiation can lead to the segregation of valuable minerals in layered intrusions.

Craig et al. (2011)

Layers of chromite (black), Bushveld (photo: Jackie
Gauntlett, blogs.agu.org)

Iron-titanium oxide (ilmenite) mining in Norway (wikipedia) Formation of oceanic crust at mid-ocean ridges

Intrusive equivalent of basalt
Solid mantle rock!!!

Understanding Earth 6th Ed.

Understanding Earth 6th Ed.

Large ophiolite outcrops
occur in Oman

www.omantourism.gov.om

100-80 x 106 yr ago
Subduction
Cont. litho.

Obduction

Oceanic litho.

Oman

Copper sulfide deposits
related to black smokers
Modified from Schreurs and Milson (2006)